This invention relates to a method and apparatus for controlling injection compression molding wherein thermoplastic resin is injected while metal molds are opened slightly and then the metal molds are clamped together, and more particularly a method and apparatus for controlling a mold clamping force or pressure.
In the art of plastic molding, it is necessary to minimize as far as possible the stress remaining in the molded product and to completely reproduce the complicated configuration of the mold cavity. In most cases, reproducibility is considered essential. This is also true in the art of injection compression molding.
When compared with a conventional injection molding wherein a hold pressure applied to a molten resin after it has been injected into a mold cavity, the injection compression molding is more advantageous in that generation of a stress especially at a portion near a gate is small so that the stress created in a molded product as a whole is small, but in a molding machine in which high pressure is stepwisely applied to the injected resin for improving the reproducibility, at portions of the molded product where the thickness thereof varies or at a boss of the product, generation of the stress is unavoidable. In an extreme case, the stress of the molded product as a whole becomes large, thus causing flexture of the product and decreasing the accuracy thereof.
For this reason, as shown in FIG. 12, it has been the practice to stepwisely or gradually decrease the mold clamping pressure (without any pulsation) as the molten resin injected into the mold cavity shrinks due to cooling and solidification of the molten resin.
With this method, however, the stress can be alleviated, since the mold clamping pressure is successively decreased without pulsation, transmission of the pressure is not sufficient thereby decreasing the reproducibility.